Sir Edmund's sister Elizabeth was married to Henry "Hotspur" Percy. Sir Edmund made an arrangement with the Percy family of northern England and Owain Glendower of Wales (now Sir Edmund's father-in-law) called the Tripartite Indenture, by which they would overthrow Henry and divide Great Britain among themselves. Their first attempt to overthrow Henry IV resulted in the death of Hotspur.
They did not give up. One step toward success was gaining control of the "captive" Edmund. Henry was raising Edmund and his brother Roger with Henry's own younger children at Windsor Castle.
For the rescue they turned to Constance of York, whose husband had been executed after a plot to assassinate Henry IV. In February 1405, Constance and her brother, Edward of Norwich Duke of York, managed to get to Edmund and Roger and kidnap them. They were overtaken in Cheltenham, 3/4 of the way to Wales.
Constance and Edward of Norwich were imprisoned. Many monarchs would have killed the buy in order to prevent him from being the focal point for a rebellion. Henry IV was kinder than that, as well as confident of his military superiority that could put down any rebellion.
Edmund and Roger were returned to Windsor, but ultimately kept in the Tower of London. Sir Edmund was kept in check and died in 1409. With the chief rebel gone, Henry IV handed custody of young Edmund to the Prince of Wales, Henry (later King Henry V), who was 22, five years older than Edmund.
While Henry IV lived, he would not allow Edmund or Roger to go free. They were now old enough and savvy enough to understand that they might someday become king of England with the right backing, and there were still people in England who would support them. Tomorrow I'll explain their fate after Henry IV died. You might be surprised.
